The liquefaction of natural gas for storage and transportation and regasification for final distribution is a well established technology. Liquefied natural gas (LNG) represents an economically attractive energy option, especially for industrial nations short on domestic fuel reserves.
Several types of natural gas liquefaction processes are known. One conventional LNG process, the standard cascade system, uses three different refrigerants, i.e., methane, ethylene and propane, circulating in closed cycles. One example of such a system is described in U.S. Pat. No. 3,593,535. An improvement over the standard cascade system employs a single-pressure mixed refrigerant cascade (MRC) system. In one version of the MRC system described in Geist et al., "Predicted and Actual Temperature Profiles and Pressure Drops in Large Coil Wound, Mixed Refrigerant Heat Exchangers," LNG6, Session II, Paper 4, April 7-10, 1980, Kyoto, Japan, a natural gas feed following treating and drying is precooled in an auxiliary heat exchanger supplied with propane refrigerant. Thereafter, the chilled gas is introduced into a cryogenic main heat exchanger (MHE) where liquefaction takes place. The MHE is horizontally divided into an upper cold bundle absorbed by propane and a lower warm bundle absorbed by mixed refrigerant.
The industry, in general, is predominantly reliant on propane as a refrigerant. Such reliance poses serious problems, especially from the standpoint of obtaining very low temperatures. It is quite difficult to provide very low temperatures by use of propane. Propane as a refrigerant is limited by its boiling point at a workable pressure of about 20 psi.
Regardless of the liquefaction system used, aluminum is often the material of choice for the construction of the cryogenic heat exchanger due to its high thermal conductivity, excellent low temperature properties, machinability and relatively low cost. However, aluminum is susceptible to corrosion by the mercury which is present in natural gas, e.g., from as low as about 0.005 to as high as about 200 micrograms per normal cubic meter (i.e., from about 5.5.times.10.sup.-3 to about 220 parts per billion by volume). Concentrations of mercury greater than about 0.01 micrograms per normal cubic meter are generally regarded as undesirable especially where aluminum cryogenic liquefaction equipment is concerned due to mercury's capability for forming a corrosive amalgam with aluminum. This type of amalgamation weakens the aluminum heat exchanger by creating cracks which can ultimately result in explosions of the higher pressure vessels.
Although it is a conventional practice to demercurate natural gas (see, for example, the demercuration processes described in U.S. Pat. Nos. 3,193,987; 3,803,803; 4,101,631; 4,474,896; 4,491,609; 4,474,896; and, 4,500,327), a sufficient amount of mercury will often remain in the post-treated gas as to pose a significant safety and maintenance problem where aluminum cryogenic heat exchangers are concerned. In its solid state, however, i.e., below its point of solidification, mercury does not tend to form an amalgam with aluminum. While this fact might have been considered useful in preventing corrosive mercury-aluminum interaction, the industry's reliance on propane as a refrigerant has until now limited the ability to attain the temperature required to solidify the mercury in mercury-containing natural gas streams.
Thus, it is an object of the present invention to provide a process for the liquefaction of a mercury-containing gas, in particular, natural gas, in aluminum cryogenic equipment in which the corrosion potential of the mercury for the aluminum is lessened or minimized.
It is a particular object of this invention to reduce the corrosion potential of elemental mercury for aluminum cryogenic gas processing equipment by exploiting the observation that at or below its freezing point, mercury exhibits no appreciable tendency to form an amalgam with aluminum, specifically, by introducing the mercury-containing feed gas into the main heat exchanger (MHE) at a temperature at which the mercury is present in the gas in the solid state.